Prior technical solutions have been based predominantly on ceramic masses of the perovskite structural type having the general formula ABO3, wherein the piezoelectric properties are brought to bear in the ferroelectric state. Lead zirconate titanate ceramics that have been modified using select additives, Pb(Zr1−xTix)O3=PZT, the composition of which is tailored to the so-called morphotropic phase boundary of two coexisting ferroelectric phases, have proven particularly advantageous. Between the ceramic layers, which are produced according to methods customarily used with ceramics, especially ceramic foil technology, precious metal internal electrodes, for example Ag/Pd in a 70/30 molar ratio, are applied by means of the screen printing process. With up to several hundred electrode layers per component, these components are burdened with substantial costs. The precious metal electrodes enable the thermal elimination, in air, of dispersants and binders and other organic additives, along with the organic components of the screen printing metal paste, from the multilayer stacks by means of depolymerization and oxidation, thus enabling a subsequent sinter densification at ca. 1100 to 1150° C. without reduction effects that could, for example, be caused by residual carbon, which would negatively influence the properties of the ceramic as a result of reduction reactions.
In the publication DE 20023051 U1 a method for producing piezoelectric components is disclosed, which uses copper-containing electrodes in place of the costly Ag/Pd internal electrodes, wherein the piezoelectric ratings are based upon ceramic masses having the preferred composition Pb0.97Nd0.02γ0.01 (Zr0.54Ti0.46)O3. The symbol “γ” stands for a vacancy in the crystal lattice. Ceramic masses having this type of composition are particularly well suited for use in Ag/Pd internal electrodes and for air sintering at 1120° C., and are tailored with respect to their piezoelectric properties such that they partially take up silver from the internal electrodes. The acquisition of the silver is made possible by the presence of atmospheric oxygen during the sintering process. At the same time, grain growth is promoted, so that in the finished component a ceramic composition Pb0.96Nd0.02Ag0.02(Zr0.54Ti0.46)O3 results, with a grain structure that is favorable for the intended application.
In contrast, the multilayer piezoelectric components that have the same initial composition as the ceramic and copper-containing internal electrodes do not have this type of silver content, with the result that the morphotropic phase boundary that is advantageous for optimal piezoelectric properties is no longer present in the ceramic, and the average grain size is smaller. The latter is primarily also a result of the lower sintering temperature of ca. 1000° C., which must be maintained when internal electrodes containing copper are used, in order to prevent a melting of the electrodes.
Although, with the sintering of multilayer components based upon the PZT ceramic of the composition Pb0.97Nd0.02γ0.01(Zr0.54Ti0.46)O3 with Ag/Pd internal electrodes in an air atmosphere at 1120° C., the silver becomes inserted evenly over the entire cross-section of the sintered ceramic layer, so that the composition Pb0.96Nd0.02Ag0.02(Zr0.54Ti0.46)O3 is established in the piezoceramic, with the sintering of a multilayer ceramic component that has copper-containing internal electrodes the copper content in ceramic layers having the above-named composition amounts to only ca. 0.1 m %.
The deviation from the morphotropic phase boundary is perceived for example in a lower dielectric constant ∈ and in an increase in the temperature coefficient TK∈ of the DK (measured, for example, between −20° C. and 60° C., ascending) and also in a lower degree of deflection S3 at the same field intensity E3 (DK=dielectric constant).
The deflection parameter d33 (=piezoelectric charge constant) is defined by the equation S3=d33·E3. Furthermore, in the assessment of the suitability of a piezoceramic for use in multilayer components the dielectric loss L is the critical factor, which, as a function of the electric activation, causes a more or less significant temperature increase in the component and can be described using the degree of efficiency η=Ea/Ee (Ea=energy that can be coupled out, Ee=coupled-in energy) combined with the electric field intensity that is associated with a certain degree of deflection E=U/d (d=thickness of the ceramic layer), by means of the equation L=(1/2)U2C(1−η) (C=capacitance).